LAUSR

A seismic Benioff zone and plate kinematics show Puysegur Trench south of New Zealand transitioning to subduction. Because the local structure and its influence on subduction initiation is poorly understood, we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles. Our early results show that the overriding Pacific Plate beneath the Solander Basin is composed of block-faulted and thinned continental crust, and the inner trench wall of northern Puysegur Ridge is composed of folded and faulted sediment. The megathrust interface has been imaged and shows ∼500 m of downgoing, undisturbed sediments. Combining plate kinematic history with seismic velocity-inferred density, we show that the density difference across the plate boundary changed as oblique strike-slip plate motion juxtaposed dense oceanic crust with thinned continental crust. The density difference rapidly increased 18 to 15 Ma, coincident with subduction initiation, suggesting that compositional differences have a large influence on subduction initiation.